On Monday, October 12th, 2009, Twitter communicated that recurring tweets are in violation of their Terms of Service. Twitter's rationale centered around the potential for recurring tweets to result in duplicate tweets.
From Twitter ... Recurring Tweets are a violation no matter how they are done, including whether or not someone pays you to have a special privilege. We don't want to see any duplicate tweets whatsoever- They pollute Twitter, and tools shouldn't be given to enable people to break the rules. Spinnable text seems to just be a way to bypass the rules against duplicate updates and essentially provides the same problems.
For those of you using services such as Social Oomph (previously named Tweetlater), you can still schedule tweets for a specific day/time, just not recurring Tweets.
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